The service life requirements of spark plugs for internal combustion engines are increasing steadily, since manufacturers often strive for replacement intervals of 60,000 km to 100,000 km in motor vehicles. Such replacement intervals can be achieved, at least in the case of the conventional triple-electrode spark plugs, only by using noble-metal alloys such as platinum alloys or iridium alloys in the electrode area, in particular the center electrode, and applying or attaching these alloys to the electrodes, i.e., nickel-alloy electrode materials, commonly used by extrusion, plating, resistance welding, laser welding or laser alloying. However, these methods of joining the noble metal alloy to the nickel alloy require highly sophisticated process engineering techniques, since the properties of platinum, and especially iridium, alloys differ enormously from those of nickel alloys in terms of their melting and boiling points as well as thermal expansion coefficients. In addition, preforms, such as pins made, in particular of iridium alloys, are very expensive to manufacture, due to their low ductility.
A spark plug for an internal combustion engine, which has a center electrode made of an electrode base element and a noble metal tip that is attached to the end face of the electrode base element facing the combustion chamber, is described in European Published Patent Application No. 0 785 604. The end section of the electrode base element on the combustion chamber end is also in the shape of a frustum. The noble metal tip according to European Published Patent Application No. 0 785 604, is further applied to the electrode base element by laser welding or resistance welding and is made of a platinum alloy or an iridium alloy, while the electrode base element is made of a nickel alloy with a core made of a heat-conductive material.
The design of the noble metal tip in the shape of a frustum is also described in German Published Patent Application No. 100 11 705. This publication further describes the use of a metal alloy, with ruthenium as the primary component, as a spark erosion-resistant electrode material for spark plugs.
An electrode material in the form of a metal alloy that is particularly suitable for use in spark plugs is described in European Published Patent Application No. 0 866 530. This material is a metal alloy with iridium as the primary component and additional noble metals, such as rhodium, ruthenium or rhenium, as secondary components.
It is thus conventional that iridium alloys and ruthenium alloys are suitable for use as electrode materials in spark plugs, due to their extremely high melting points and associated erosion resistance. A process is also described whereby preferably rhodium is added by alloying to iridium, due to the latter's poor oxidation stability. However, alloys of this type are very brittle and thus very expensive to work, which means that the manufacture of preforms, such as pins or disks, which are subsequently to be joined—particularly by welding—to conventional electrode base elements made, for example, of nickel, is very costly.